One of the major expenses in a telephone system is that of providing a line interface circuit for connecting each and every telephone line at a central office switching facility, or at a private branch exchange facility. In any practical telephone system, the line interface circuits are desirably inexpensive and essentially electrically robust. During those times when a telephone set is in use, the associated telephone line interface circuit provides an energizing direct current for the telephone set, via tip and ring leads of the telephone line. When the telephone set is in use, it is usually referred to as being OFF HOOK, and when the telephone set is not in use, it is referred to as being ON HOOK. The line interface circuit is also required to provide indication as to the telephone set being ON HOOK or OFF HOOK.
A source of the energizing current is usually a central battery supply. The central battery supply is continually charged to a predetermined voltage by a battery charging apparatus, provided for that purpose. The battery charging apparatus is operated from utility power, but in the event of an interruption of the utility power the central battery supply is intended to continue to supply operating current for a limited time. The battery charging apparatus often includes a switching invertor, which as a by-product of its operation, generates electrical noise. Typically one of the functions of the line circuit is that of preventing noise signals of any origin from traversing the connection between the central battery supply and the telephone line. For example, in the U.S. Pat. No. 4,103,112 issued on Jul. 25, 1978 to V. V. Korsky, and titled "Telephone Line Circuit With Differential Loop Current Sensing And Compensation", a telephone line is terminated at tip and ring terminals connected to tip and ring windings of a transformer. Energizing direct current is conducted by 200 ohm tip and ring feed resistors connected in series between the battery supply and the tip and ring windings. A 2.16 microfarad capacitor is connected between the tip and ring windings. The capacitor provides an impedance path for alternating current voice band signals, whereby very little of the signal energy is dissipated across the feed resistors.
The value of the capacitor is essential in determining the terminating impedance of the line circuit. In U.S. Pat. No. 4,864,609, issued on Sep. 5, 1989 to M. S. Moisin, and titled "Telephone Line Interface Circuit", a compensating circuit for adjusting the value of the operating terminating impedance to more closely approach the value of a predetermined preferred terminating impedance, is discussed. The compensating circuit is connected to drive a winding, in the transformer, in a current opposing relationship with respect to some of the differential alternating current signals in the tip and ring windings. The differential alternating current signals specifically referred to are those within a lower portion of the voice frequency range, for which an impedance of the capacitor is significantly shunted by the feed resistors.
U.S. Pat. Nos. 4,764,956, issued 16 Aug. 1988 to R. Rosch et al; 4,514,595, issued 30 Apr. 1985 to S. Rosenbaum et al; 4,484,032, issued 20 Nov. 1984 to S. Rosenbaum; 4,539,438 issued 3 Sep. 1985 to S. Rosenbaum et al; 4,532,381, issued 30 Jul. 1985 to S. Rosenbaum et al and 4,571,460 issued 18 Feb. 1986 to S. Rosenbaum et al are each concerned with providing the required terminating impedance via active circuit apparatus. Particularly, tip and ring amplifiers are controlled in response to direct loop current and alternating differential current signals detected across tip and ring feed resistors, to generate the terminating impedance and to inject alternating current signals for transmission via the telephone line to the telephone set. The detected line current is also used as a source of alternating current signals for transmission to a hybrid circuit wherein signals from the telephone set are transmitted via the associated telephone facility.
The performance of these types of line interface circuits is adversely affected by practical limitations of common mode rejection characteristics of analog differential amplifier circuits. These characteristics may be significantly compromised in the interest of economy, by integration of the analog differential amplifiers along with digital circuit elements, essential for pulse code modulated time division multiplex interface with the associated telephone facility. These characteristics may be further compromised if either of the tip and ring feed resistors becomes mismatched. For example, during use of a line interface circuit, one of the tip and ring feed resistors may age in response to unusual stress, as may accompany a transient electrical event such as a power line cross or a lightning strike in association with the telephone line.